1. Field of the Invention:
This invention relates to a linear actuator for linearly driving an output shaft with electromotive force, and more particularly to a linear actuator adapted to allow an electric current supplied to a coil to be efficiently converted into an electromagnetic force.
2. Description of the Prior Art:
The so-called voice coil type linear actuator is used in generating a motion of a relatively large stroke as in driving an exhaust gas recirculation valve or an air conditioning valve mounted in an automobile.
This actuator comes in two types, one type comprising a movable magnet formed integrally with an output shaft and a coil disposed stationarily within the magnetic field of the magnet and the other type conversely comprising a magnet disposed stationarily and a movable coil formed integrally with an output shaft. In the actuator of either of the types, when the coil is energized, the electromagnetic force consequently generated operates the coil and the magnet relative to each other and causes the output shaft to produce a linear motion.
One example of the linear actuator utilizing the above operating principle is disclosed in Japanese Utility Model Laid-Open Publication SHO 61(1986)-117,588.
FIG. 2 represents a cross section of a prior art movable magnet type actuator having a magnet and an output shaft integrated with each other, one of the two types of actuators mentioned above.
With reference to the diagram, an output shaft 1 is inserted in the central part of a cuplike magnet retaining member 2 made of an electrically insulating material such as resin and joined integrally with the retaining member 2 by a nut 3. To the outer surface of the retaining member 2, a magnet 4 is attached fast with adhesive agent.
A movable part 16 composed of the output shaft 1, the cuplike member 2, and the magnet 4 is adapted to be reciprocated as guided on the outer surfaces of a cylindrical guide member 7 and a bearing 6 installed in a casing 5. The guide member 7 is supported in the casing 5 with a supporting plate 7a.
One end of a coil spring 8 is engaged with a screw member 9 that is screwed into the center bore of the guide member 7 so as to permit adjustment of the amount of the strain given in advance to the coil spring 8. The other end of the coil spring 8 is kept in engagement with a projection at the bottom of the magnet retaining member 2.
As the result, the resilient force of the coil spring 8 presses the retaining member 2 in the direction of the bearing 6. The pressing force generated as described above by the coil spring 8 can be adjusted by moving the screw member 9 forward or backward in the axial direction of the spring 8 and the output shaft 1.
A coil 12 is positioned in the vertical direction by retaining plates 10a, 10b so as to encircle the magnet 4 in an open space between the casing 5 and the guide member 7.
The coil 12 is connected at one terminal thereof to a lead terminal 14 and at the other terminal to the other lead terminal (not shown). The lead terminal 14 is fixed in a terminal fixing plate 15 made of an electrically insulating material.
In the actuator constructed as described above, when an electric current is supplied to the coil 12, the electric current and the magnetic flux of the magnet 4 passing through a magnetic circuit 13 interlink to generate an electromagnetic force, by virtue of which the magnet 4 and consequently the movable part 16 are moved in the direction of compressing the coil spring 8.
As the movable part 16 is moved, the spring 8 is compressed more and more to increase a resilient force. The movable part 16 is brought to a stop at the position at which the electromagnetic force and the resilient force of the spring 8 are balanced. The amount of movement of the output shaft 1 from the position of rest assumed when no electric current is supplied to the coil 12 to the position of balance assumed when the movement of the movable part 16 is brought to a stop during the supply of electric current to the coil 12 constitutes itself the stroke of the actuator. The stroke of the actuator, therefore, is fixed by the magnitude of the electric current supplied to the coil 12 and the resilient force of the spring.
The conventional technique described above entails the following drawbacks.
The magnitude of the electromagnetic force generated by the electric current supplied to the coil 12 is dependent on the values of permeability of the component members of the magnetic circuit 13, namely the casing 5, the guide member 7, and the supporting plate 7a for the guide member 7. To be specific, the generation of the electromagnetic force by the supply of the electric current to the coil 12 is attained efficiently in proportion as the magnetic resistance of the magnetic circuit 13 is decreased and the magnetic flux of the magnet 4 is consequently increased. For the sake of the efficient generation of the electromagnetic force, the component members of the magnetic circuit 13 are desired to have large permeability.
Incidentally, the component members of the magnetic circuit 13 are manufactured by machining proper blanks in desired shapes. The materials for these component members, therefore, are required to possess satisfactory machinability. Particularly since the casing 5 has large dimensions and a complicated shape, the material used therefor is desired to possess highly satisfactory machinability.
It is, however, difficult to select from among various magnetic materials a particular material which simultaneously meets the requirements, i.e. high permeability, highly satisfactory machinability, and low cost. It has been inevitable to select the material for the casing at a sacrifice of either machinability or permeability.
The present invention has been produced for the purpose of solving the disadvantage described above.